The present invention relates to a cooling system used with a printed circuit board holding a number of solid electronic circuit components, such as integrated circuit (IC) semiconductor devices. More particularly, it relates to a cooling module, included in a cooling system, which is in contact with each of the electronic circuit components through a thermally conductive compound layer. The cooling module cools these components.
Various types of cooling structures have been developed for cooling IC semiconductor devices or large scale IC (LSI) semiconductor devices mounted on a printed circuit board as disclosed, for example, in U.S. Pat. No. 3,993,123 issued to Chue et al., 4,203,129 issued to Oktay et al., 4,254,431 issued to Babuka et al., and 4,323,914 issued to Berndmaier, and in not-examined provisionally published Japanese Patent Applications No. 61-15353 by K. D. Ostergrane et al., No. 60-160149 by Yamamoto et al. and No. 62-109347 by Tajima. In some of these cooling structures, a heat transfer element, such as a heat transfer plate or a heat transfer piston, is placed in direct contact with the circuit components. The heat transfer element is urged to contact a surface of a circuit component by pressure provided from a spring, bellows, hydraulic pressure from coolant, etc., to remove the heat dissipated from the circuit components. The heat transfer elements are exposed either directly or indirectly to a coolant. The heat is transferred to the coolant through the corresponding heat transfer elements which are in contact with the circuit components.
In general, however, a rather high and unstable heat contact resistance interface exists between the heat transfer elements and the circuit components in the above described cooling structure because the actual contacting area therebetween is rather small and unstable due to the roughness of the surfaces which are thermally in contact with each other. In addition, any slight change of the pressure of the spring, bellows, and particularly, the coolant pressure, seriously affects the heat contact resistance, resulting in a large loss of the heat transfer efficiency of the cooling modules. In particular, the use of a spring for urging a heat transfer member into contact with the corresponding circuit component to be cooled, tends to cause mechanical resonant vibration triggered by an external mechanical shock, resulting in variation of the exerting pressure.
In order to overcome the aforesaid disadvantages caused by the prior art cooling structures, various liquid thermal conductive materials such as thermally conductive inert gas, a liquid metal or thermal silicon grease, or a compliant thermal conductive material is adhesively inserted into the interface between the surfaces of the heat transfer element of the cooling module and a circuit component. For example, a thermal conductive inert gas is introduced into the interface by Chu et al., and a low boiling point liquid is utilized to immerse a heat transfer piston and a circuit component by Oktay et al. However, a complicated and costly sealing structure for sealing the gas or the liquid is required for both cooling modules. In addition, the thermal conductivity of the associated inert gas and liquid set the reduction of the relevant thermal contact resistance within an upper limit.
Berndmaier et al and Babuka et al both employ a liquid metal or alloy to fill up a contact interface. Ostergrane discloses using a thermal grease in the interface between the conical surfaces of a piston and a hat, and using a liquid metal layer between the piston and a circuit element. With this structure, a rather thick layer of the thermal grease may be required to maintain the layer on the conical surface of the piston, which causes an undesirable increase in the thermal resistance of the layer. In addition, some types of liquid metals usually have a probability of chemical reaction with the contacting material, which requires various counter measures to prevent the reaction.
Yamamoto et al insert a compliant sheet between a circuit component and a heat transfer plate which is urged toward the circuit component by a bellows to reduce thermal contact resistance across the interface between the component and the heat transfer plate. However, a relatively thick compliant sheet is required in order to realize a desirable perfect thermal contact between the sheet surfaces, the contacting surfaces of the relevant circuit elements and the heat transfer plate by expelling small air voids remaining on the contacting surfaces. As a result, the reduction of the whole thermal contact resistance across the interface is adversely affected.
Tajima provides a cooling structure comprising a cap having a spherical top surface, a stud having a concave spherical bottom surface engageable with the spherical surface of the cap, and a cooling hat. The cap contacts with a circuit element with a small gap therebetween which is filled with thermal grease. The stud is secured to a cooling header. The cap and the hat are in contact with each other through a layer of thermal grease which has a considerable thickness sufficient to protect the circuit element from being subjected to pressure. Tajima discloses nothing about pressure to be exerted on the thermal grease layer.
In these prior art cooling modules, a large effort has been made to reduce the heat transfer resistance across a thermal contact interface utilizing various thermally conductive compound materials. However, the results are not sufficient to maintain a desirably low, stable and reproducible thermal contact resistance.